A. Field of the Invention
This invention relates to call routing in a network access server. More particularly, it relates to a method by which a land-side originated call is routed to one of a plurality of modems in a network access server. The network access server provides an interface between a land-side multiplexed digital network (e.g., ISDN primary rate interface connected to the PSTN) and a wireless network (e.g., frame relay line in a wireless network that is used to connect the call with a mobile communications device, e.g., a laptop computer with a cellular telephone modem or cellular phone, via a mobile switching center and base station).
B. Description of Related Art
A network access server is an integrated communications device that is used to demultiplex channels of a multiplexed digital telephone line, perform signal conversions on data contained in the channels with a plurality of modems, one modem per channel, and route the data onto a second network via a gateway interface card or module. A representative network access server is described in the patent to Dale M. Walsh, U.S. Pat. No. 5,528,595, which is incorporated by reference herein. A network access server suitable for coupling a multiplexed frame relay line of a wireless network to a packet-switched IP or other network is described in the patent application of Yingchun Xu, Ser. No. 08/887,313, also incorporated by reference herein. Network access servers are also available from 3Com Corporation, i.e., the Total Control Enterprise Network Hub. Similar devices are available from other companies in the industry, including Lucent Technologies (successor to Livingston Enterprises), Ascend Communications, Multitech, and still other companies.
When a network access server is used as an InterWorking Function (IWF) unit to couple a wireless network to a public switched telephone network, problems can arise in routing a call through the network access server, particularly where a land-side call is placed to a mobile wireless device (a situation referred to herein as a xe2x80x9cmobile terminatedxe2x80x9d call). A problem arises in that when the land-side telephone wishes to place a call to the mobile node, the call goes through the Public Switched Telephone Network (PSTN) to a mobile switching center which then pages the mobile node through the wireless network. Meanwhile, a connection over the wireless network to the network access server/IWF is initiated and the call is assigned to one of the modems in the IWF. When the land side part of the call comes into the IWF, the land-side network interface card or module may not be able to determine which port or modem in the IWF is holding on to the wireless side of the call. Unless the two halves of the call (PSTN to IWF, and Mobile to IWF) can be put together at a single modem in the IWF, they will not connect and the call will be dropped.
The present invention addresses this situation and provides a way to insure that mobile terminated calls are efficient and correctly routed in the IWF such that a single modem in the IWF is assigned to both halves of the call.
A method is provided of routing a call through a network access server having a plurality of modems, a first interface module connecting the network access server to a first network (e.g., PSTN) and a second interface module connecting the network access server to a second network (e.g., a wireless network). Each of the modems are associated with a modem channel in the network access server. The method comprises the step of transmitting information to the first interface module regarding the routing of calls from the second interface module to the individual modems. The first interface module uses this information to route a call received from the first network to one of the modems. For example, the first interface module connected to the PSTN network may maintain a table mapping the allocation of particular calls to modem channel numbers and modem slot numbers, (e.g. by called number or calling number, or both). The table entries are populated by information received from the modems which identify the modem by slot and channel numbers and information identifying the call (e.g., called telephone number or a unique call reference value). Thus, when the land side of the call comes in to the network access server at the first interface module, the interface module can look to the table to determine which modem channel number and slot number to assign to the call. Accordingly, a connection between an endpoint on the first network and an endpoint on the second network may be achieved and the call routed via the modem assigned to the call in an efficient manner.
In a preferred embodiment, the information received by the first interface module comprises (1) a called party number and (2) information identifying a modem in the plurality of modems, e.g., by slot number and channel number. A further possibility is that the information comprises a call reference value uniquely identifying the call and information identifying a modem in the plurality of modems.
In a preferred embodiment, the first interface module maintains a routing table storing called party numbers for a plurality of calls and that associates each of the called party numbers with a modem slot number and a modem channel number. Ideally, the table is continually updated as new calls come in, and calls are dropped. Then, whenever a new call comes to the first interface module (e.g., PSTN) the module can look to the routing table and correctly match up the call to a modem in the IWF, simply by looking at the called party number in the new call and using the routing table to determine which modem to send the call to.
In another aspect of the invention, an improvement to a network access server is provided. The network access server comprises a first interface module, a plurality of modems, a second interface module and a bus complex linking said first and second interface modules and said modems together. The improvement comprises the feature of implementing a table in the network access server that maps called party numbers to modem numbers in the network access server. The table is used to route a call received at the respective interface module to a modem designated to receive the call. Preferably, the table is maintained in the interface module that is the second interface to receive the call and needs to know where to route the call to a modem already assigned to the call.
Additional features of the invention will be more apparent from the following detailed description.